Reiserer Group

Towards a quantum internet

A future quantum network will consist of quantum processors that are connected by quantum channels, just like conventional computers are wired up to form the Internet. In contrast to classical devices, however, the information that can be encoded in a quantum network grows exponentially with the number of nodes, and entanglement of remote particles gives rise to non-local correlations. Exploring these effects facilitates fundamental tests of quantum theory and the quantum-to-classical transition. In addition, quantum networks will enable applications in precision sensing and in distributed quantum information processing, which will fundamentally enhance computational power and ensure unbreakable encryption over global distances.

Pioneering experiments with atomic ensembles, single trapped atoms and solid-state spins have demonstrated the connection and entanglement of two quantum nodes separated by up to 1.3 km. However, accessing the full potential of quantum networks requires scaling of these prototypes to more network nodes and even larger distances. To this end, a new technology that overcomes the bottlenecks of existing physical systems has to be developed.

In this context, we employ the spin of individual rare-earth ions embedded into thin crystals. In contrast to other impurities, rare-earth ions exhibit optical transitions between inner-shell 4f levels. Like in a Faraday cage, the electrons in these levels are largely shielded from the electric fields of the crystal by the outer shell 5s and 5p electrons. When operating at a specific magnetic field, also the spin transition frequencies can be made insensitive to magnetic fluctuations. In this way, the current world record of six hours of quantum coherence time has been achieved (using Eu3+ in Y2SiO5 crystals in M. Sellars’ group, ANU Canberra).

The remaining challenge in the context of quantum networks is that the long-lived optical transitions of rare-earth impurities typically lead to a small fluorescence rate, which makes optical detection and readout challenging. To solve this task, we will embed rare-earth-doped crystals into optical resonators. In this setting, the excited state lifetime can be strongly reduced by the Purcell effect, and the emission of individual ions is efficiently channeled into an optical output mode.

Our main focus lies on the rare-earth element Erbium because it is the only known impurity that has coherent optical transitions within the “telecommunications” wavelength regime. This allows us to reduce the experimental overhead by harnessing existing photonic technologies. In addition, loss in optical fibers is minimal at these frequencies: compared to the visible, the transmission over 100km of optical fiber is improved by about 40 orders of magnitude. This is mandatory to realize quantum networks that span global distances.